1. Field of the Invention
The present invention relates to copying machines, printers, facsimiles, multi-function machines utilizing electrophotography and a scanning optical unit, specifically to an image forming apparatus having an optical system with which light from a light source is reflected by a rotating mirror to scan a photoreceptor and a scanning optical unit used for the apparatus.
2. Description of the Related Art
In an image forming machine such as a copying machine, printer, facsimile, and multi-function machine of them, there are demanded high image quality, high speed image forming, compactness of the apparatus, and low cost of the apparatus. Therefore, it is required to form images of high quality, reduce time required for image forming per sheet, simplify the construction of the apparatus, and reduce the number of parts constituting the apparatus.
Among them, as to increase in quality, fineness of pixels has been improved by minimizing the diameter of a light flux emitted from a light source. As to speeding-up of image formation, an image forming apparatus is proposed in which image forming time is reduced by allowing light beams from a plurality of light sources to be imaged on a photoreceptor in circumferential direction (hereafter referred to as secondary scanning direction) thereof and they are traversed in axial direction (hereafter referred to as primary scanning direction) of the photoreceptor so that a plurality of scans are carried out simultaneously along the axial direction.
For example, a multibeam combining type scanning and recording apparatus is disclosed in Japanese Laid-Open Patent Application No. 2002-228959, which includes a plurality of light emitting means, a light beam combining means for allowing the light beams to travel in the same direction such as a polarizing beam splitter, a deflecting means for deflecting the light beams allowed to travel in the same direction, a focusing means for imaging the deflected light beams on a surface to be scanned, a pair of wedge prisms each fixed in each of hollow micro-step motors rotatable in counter directions to one another to deflect at least one of the light beams so that it or they are imaged on the imaging surface in a state shifted in the secondary scanning direction, and a position adjusting slit unit as a means for detecting the amount of the shift in the secondary scanning direction.
An optical scanning apparatus is disclosed in Japanese Laid-Open Patent Application No. 2004-258173, in which, as shown in FIG. 6(A), light fluxes from two light sources 70 such as laser diodes are collimated by collimators 71, fine adjustment of imaging positions of the light fluxes in the secondary scanning direction is effected by parallel glass plates 72, fine adjustment of angles of incidence of the light fluxes upon a reflection plane 75 of a rotating reflector is effected by wedge prisms 73, the light fluxes are trimmed through apertures, and one of the light fluxes is allowed to enter a prism 74 of triangular or nearly parallelogram cross section to be reflected from the internal reflection plane of the prism and changed in its light path so that the two light fluxes come close to one another and enter a cylindrical lens 77 to be focused on the reflection surface of the rotating reflector 75 at a prescribed tilt angle between the two light fluxes.
The cylindrical lens 77 used here is, as shown in perspective views of FIG. 6(B) and FIG. 6(C) of a supporting member of the cylindrical lens viewed from the incoming side and exiting side of light flux respectively, is supported by pressing both end parts of the cylindrical lens 77 with cylindrical lens presser springs 79 and 80 provided on a supporting frame 81 of the cylindrical lens 77 so that the light fluxes can pass through the central area of the cylindrical lens 77 between the presser springs.
However, in the multibeam combining type scanning and recording apparatus disclosed in said Japanese Laid-Open Patent Application No. 2002-228959, a λ/2 plate is used together with the polarizing beam splitter, so each of the light fluxes is reduced in half in light intensity on the polarization plane of the beam splitter. Therefore, it is necessary to increase light source intensity. It is also required to use light sources of stronger intensity when speeding up image forming speed because the time period for scanning the surface of the photoreceptor is reduced. When light intensity is reduced at the beam splitter, further stronger light sources are required, and electric power consumption increases and at the same time a countermeasure for heat generation of the light sources becomes necessary.
With the optical scanning apparatus disclosed in said Japanese Laid-Open Patent Application No. 2004-258173, attenuation of light by the prism and reflection mirror is small, so large attenuation in light intensity does not occur as does when the beam splitter is used, and strong light sources which require a strong power source are not required. But, in this case, an optical element (for example, the cylindrical lens 77) for concentrating light fluxes from each of the light sources 70 on the reflection plane 75 of the rotating reflector is required to be provided for each of the light sources or a lens of large aperture is required when each of the light fluxes is departed from one another (that means that the crossing angle of the incoming light fluxes to the reflection plane is large). Therefore, it is desirable that the light fluxes are allowed to be as close to one another as possible.
However, when the crossing angle is too small, there occurs a possibility that a corner of the prism 74 which is a light reflecting means interferes with the light flux traveling linearly and eclipses the light flux partly, resulting in that a desired light spot is not obtained on the scanning surface. Therefore, it is preferable that the crossing angle is such that the eclipse by the prism 74 does not occur, but for this is required a larger aperture of the cylindrical lens 77.
As has been explained referring to FIGS. 6(B) and (C), the cylindrical lens supporting member 78 for supporting the cylindrical lens 77 is composed such that the both end parts thereof in the longitudinal direction are held by the presser springs 79 and 80, the cylindrical lens has to be widened in the longitudinal direction in order to secure portions to hold the lens with the lens presser springs. Therefore, space for providing the cylindrical lens is increased inducing increase in cost. It is thinkable to provide a cylindrical lens for each light source, but this countermeasure results in increase in the number of parts and cost.